A programmable logic controller (PLC) is an electronic device connected to various devices so as to systematically control these devices. A PLC system uses a programmable memory to perform a special function such as logic, sequencing, timing, counting or computation through a digital or analog input/output module, and controls various types of machines or processors.
Generally, the PLC system is one of the most basic and important control systems together with industrial automation, and is frequently used in facility automation of factories, buildings, etc. The PLC system currently satisfies various spot requirements, not only by performing complicated application computation from initial basic computation, but also by forming industrial networks. As new up-to-data devices are introduced to the PLC, an extension PLC system has been recently introduced, in which a plurality of extension bases is connected in addition to a main base to control a more complicated network.
While productivity increase has become an importance factor on industrial spots, interest in safety for reducing human and material damage in the operation of the complicated PLC system has recently increased. Particularly, trends are regulations and requirements for safety are further deepened. According to such requirements, it is important, more than anything else, to reduce material damage caused by malfunction and human damage according to the material damage by controlling the PLC system to operate each of the plurality of extension bases connected to the main base.
FIG. 1 is a configuration diagram of a conventional single PLC system.
Referring to FIG. 1, in the conventional single PLC system, a plurality of extension bases 11 is connected to a main base 10 through communication cables 12. Each extension base 11 is controlled by the main base 10 and operates depending on the state of the main base 10. Thus, the extension base 11 has a structure in which the extension base 11 is passively operated by the main base 10.
FIG. 2 is a configuration diagram of a conventional redundant PLC system.
Referring to FIG. 2, the conventional redundant PLC system has a structure in which a main base 20 is divided into a master main base 21 and a standby main base 22. Each extension base 23 is connected to any one of the master main base 21 and the standby main base 22 through a communication cable 24. Similarly to FIG. 1, each extension base 23 is controlled by the main base 20, and is passively operated by the main base 20. In a case an error occurs in the master main base 21, the standby main base 22 takes over control authority from the main base 21 so as to control each extension base 23.
However, in a case a defect occurs in the conventional PLC system, the main base recognizes the occurrence of the defect and provides the state of the system to the extension bases. This is because the system has a dependent structure in which the extension bases are controlled according to the presence of recognition of the main base with respect to the defect and the recognizable performance of the main base. Therefore, in a case a defect occurs in the conventional PLC system, the main base does not recognize or slowly recognizes the occurrence of the defect, and hence the state of the system is not properly provided to the extension bases. Accordingly, malfunction may occur in devices mounted to the extension bases, and human and material damage according to the malfunction may be caused. Since a control module of the extension base has a dependent structure of the main base, the control module of the extension base does not recognize the state of the system for itself. Therefore, the extension base does not independently operate, and accordingly, the system is exposed to risk cause by the malfunction.